The present invention relates to respiration sensing equipment and in particular to a respiration sensing device suitable for monitoring respiration during radiation therapy during breath-hold protocols and synchronized breathing protocols.
Accurate monitoring of a patient's breathing can be important in a wide variety of medical applications. In imaging, an accurate knowledge or respiration phase may be used to properly assemble x-ray tomographic or magnetic resonance imaging components acquired during breathing into an image free of image artifacts. In radiation therapy, accurate knowledge of respiration phase may be used to steer or limit the radiation beam to accurately apply radiation to the correct tissue. In a new radiation treatment delivery described in application U.S. Ser. No. 10/702,810 filed on Nov. 6, 2003 and hereby incorporated by reference, accurate knowledge of the respiration phase is used to synchronize the treatment plan phase and respiration phase allowing continuous breathing by the patient during treatment to produce an imaging and radiation treatment.
Current systems for monitoring respiratory motion include: (1) chest displacement sensors that track the surface of the abdomen by measuring the position of a reflective marker on the chest with a fixed camera or by measuring the distance from a fixed point to the surface of the abdomen using a laser-based distance sensor, (2) spirometers measuring air flow into and out of the patient's lungs, and (3) internal markers placed on tissue of interest and monitored using x-rays, magnetic fields, or the like.
Although invasive internal markers provide the most reliable method of target position tracking, convenient non-invasive chest displacement or flow measurement spirometers are widely used in a radiotherapy clinic. Yet, these latter systems have significant disadvantages. Chest displacement systems are strongly affected by variations in set-up. Radiation treatment can extend over many sessions so it is not easy to reproduce the same measurements over the entire course of treatment. Spirometer systems measure air flow only and this signal must be integrated to obtain air volume as a function of time such as provides a measure of respiration. Slight errors in flow measurement accumulate in time and cause signal drift. Generally, spirometers exhibit a nonlinear response to air flow.
When the respiration signal is used for determining precise positioning of tissue for imaging or radiation treatment, set-up sensitivity and drift are significant problems.